Synthetic aggregates

ABSTRACT

A method of producing a synthetic aggregate, comprising preparing a finely divided raw material having an oxide analysis SiO2 5-35 percent, Al2O3 15-70 percent, FeO 0-65 percent. MgO 050 percent, alkalis &lt;5 percent. and CaO&lt;5 percent, the combined content of FeO and MgO being sufficient to provide at least 15 percent of spinels in the finished aggregate, agglomerating the raw material, and firing the agglomerated material at a temperature causing recrystallization of the raw materials until substantial elimination of any uncombined magnesium oxide is achieved and a material is produced comprising spinel crystals imbedded in a matrix of alumino-silicates, silicates, or a mixture thereof, and having a porosity low enough to provide an aggregate abrasion value less than 20 percent.

United States Patent 1191 Murray Jan. 15, 1974 SYNTHETIC AGGREGATES2,912,340 11/1959 Pincus 106/62 Inventor: Ransome James ay, 3,060,04010/ 1962 Bnstow 106/62 Gravesend England Primary Examiner-James E. Poer[73] Assignee: The Associated Portland Cement Att0rneyBaldwin, Wight &Brown Manufacturers Limited, London, England 57 ABSTRACT [22] Filed:Jan. 6, 1971 A method of producing a synthetic aggregate, com- [30]Foreign Application Priority Data Jan. 16, 1970 Great Britain 2303/70[52] U.S. Cl. 106/288 B, 106/60, 106/62 [51] Int. Cl C08h 17/04 [58]Field of Search 106/60, 62, 67, DIG. l, 106/288 B [5 6] References CitedUNITED STATES PATENTS 2,045,494 6/1936 Riddle 106/62 2,224,498 12/1940Yoshiki 106/62 2,833,659 5/1958 Bauer l06/DlG. l

prising preparing a finely divided raw material having an oxide analysisSiO 5-35 percent, A1 0 15-70 percent, FeO 0-65 percent. MgO 0-50percent, alkalis 5 percent. and Ca0 5 percent, the combined content ofFeO and MgO being sufficient to provide at least 15 percent of spinelsin the finished aggregate, agglomerating the raw material, and firingthe agglomerated material at a temperature causing recrystallization ofthe raw materials until substantial elimination of any uncombinedmagnesium oxide is achieved and a material is produced comprising spinelcrystals imbedded in a matrix of alumino-silicates, silicates, or amixture thereof, and having a porosity low enough to provide anaggregate abrasion value less than 20 percent.

7 Claims, No Drawings SYNTHETIC AGGREGATES This invention relates to theproduction of synthetic aggregates for use as roadstones in theproduction of skid resistant surfaces.

Natural roadstones in many cases either provide insufficient skidresistance or become polished in use with consequent reduction in skidresistance below acceptable levels. As a result increasing attention isbeing paid to the use of synthetic aggregates for this purpose.

Good results have been obtained with material containing a largeproportion of calcined bauxite, but unfortunately bauxite is anexpensive material which in many countries must be imported.

The object of the present invention is to provide a synthetic aggregatefor use asa high quality roadstone, utilizing raw materials which arereadily available at a low price.

Natural and known synthetic aggregates for use as roadstone in generalcomprise particles of a hard abrasive material bonded into a softermatrix which wears faster than the hard material, the aggregate beingproduced by firing a mixture of the selected hard abrasive material ofsuitable particle size together with suitable matrix forming materials.As mentioned above, the most suitable hard abrasive material is calcinedbauxite, which has the disadvantage of expense.

We have found that by firing suitable cheaply available raw materials ormixtures of raw material under suitable conditions of time andtemperature, sintering and recrystallization takes place with thecrystallization out of the fired material of spinels having a very highhardness, these spinels being imbedded in a softer matrix. Thus a singlefiring process both produce the desired hard abrasive material andimbeds it in a suitable matrix.

The general formula of spinels is R"O.R'2 3 in which R" includes Fe, Mgand Zn, R' includes Al, Cr and Mn. In terms of economy and physicalproperties in relation to the requirements of the present invention, thepreferred spinels are FeAl O and MgAl O According to the invention, amethod of preparing a synthetic aggregate comprises preparing a finelydivided raw material having an oxide analysis SiO -35 percent, A1 0 -70percent, FeO 0-65 percent, MgO 0-50 percent, alkalis 5 percent, and CaO5 percent, the combined content of FeO and MgO being sufficient toprovide at least 15 percent of spinels in the finished aggregate,agglomerating the raw material, and firing the agglomerated material ata temperature causing recrystallization of the raw materials untilsubstantial elimination of any uncombined magnesium oxide is achievedand a material is produced comprising spinel crystals imbedded in amatrix of alumino-silicates, silicates, or a mixture thereof, and havinga porosity low enough to provide an aggregate abrasion value less thanpercent.

Preferred raw materials have an alumina content in the range 15-40percent, the FeO content in the range 540 percent, the MgO content inthe range 5-40 percent, and the combined content of FeO and MgO issufficient to produce at least percent of spinels in the aggregateproduced.

Where the raw material contains at least a proportion of its iron inferric form, the firing step should be carried out under reducingconditions.

It is possible to replace at least part of the A1 0 content with Cr Oand/or Mn O but this will not normally be economic due to the highcommercial value of chrome and manganese ores.

The invention extends to a synthetic aggregate having an oxide analysisSiO 535%, A1 0 15-70 percent, FeO O-65 percent, MgO 0-50 percent,alkalis 5 percent, CaO 5%, comprising at least 15 percent and preferablyat least 25 percent of spinel crystals crystallized out of a matrix ofsilicates and/or aluminosilicates, and having an Aggregate AbrasionValue of less than 20 percent, and a Polished Stone Value of at least60.

The raw material used may be a high iron clay of suitable analysis, or amixture of the one hand of iron ore, ferruginous wastes, magnesite,magnesium oxide or hydroxide, carbonated magnesium hydroxide slurries,high iron clay or other material having a high FeO or MgO analysis, or amixture thereof, with on the other hand, blast furnace slag, pulverisedfuel ash, high alumina clay or other material having a high aluminaanalysis.

it will be understood that the waste, by-product and naturally occurringmaterials listed above for use as raw material are not pure oxides andtherefore cannot yield pure spinels. However, such materials, inappropriate admixture having regard to the analysis of the materials andthe phase diagram of principal oxides present, contain sufficient silicaand alumina to enable spinel formation to take place at moderatetemperatures and to provide a matrix material comprising silicates andalumino-silicates.

Either a wet or dry process can be employed to prepare the materials forfiring, these materials being comminuted if necessary and blended inappropriate proportions.

The blended material may, prior to firing, be nodulized by conventionalmeans, or mixed with a suitable binder, such as bentonite or anotherplastic clay, alginates, starch or tar oil residue, and extruded, toprovide a size range in the fired material either suitable for directuse in road-making, or suitable for such use after moderate crushing, inwhich case any fines unsuitable for road-making use may be recycled tothe nodulization stage. The fines may be fluxed with a material such assilica and fired to produce a satisfactory roadstone material.

Firing of the nodules or extruded material may be carried out using asuitable fuel in a rotary kiln, shaft kiln or on a sinter strand, atl,000 to 1,500C, the temperature employed and the length of the firingperiod employed being ascertained by experiment depending on the natureof the raw materials, the rate of spinel formation, and the necessityfor substantially eliminating any uncombined magnesium oxide present,which might otherwise cause unsoundness in the finished product, andreducing the porosity of the product to a level giving it an aggregateabrasion value of less than 20 percent and preferably not more than 15percent. The firing temperature must be sufficient to allow spinels tocrystallize out of a matrix of silicates and/or alumino-silicates.

The invention is illustrated further by the following examples:

EXAMPLE l.

A pulverised fuel ash having the analysis: SiO 44.0

percent, A1 29.4 percent, Fe O 9.16 percent, CaO 2.6 percent, MgO 2.0percent Carbon 8.40 percent was blended and pugged with a ground naturalmagnesite containing 3 percent silica so that the proportion of fly ashto magnesium oxide was 2:1. The resulting pug was decarbonated at 800C,taken to 1,380" in 45 min. and held at this temperature for 4 hours. [twas cooled in 30 min. to 1,000C and then tipped from the cruciblefurnace.

The phase composition of the material obtained was approximately;spinels (MgAl O and FeAl O,,) 30 percent, forsterite Mg SiO 52 percent,the remainder being a glassy material.

The material was produced in two batches which showed on test AggregateAbrasion Values of 15 percent and Polished Stone Values of 67 and 66.Similar results were obtained when the time at maximum temperature wasreduced to 20 minutes.

EXAMPLE 2.

The pulverised fuel ash employed in Example 1 was blended and puggedwith a precipitated magnesium hydroxide so that the proportion of flyash to MgO was again 2:1. A heating regime similar to that in Example 1was employed except that the maximum temperature was 1,400" and thematerial was held at this temperature for 20 minutes.

Two batches were prepared which showed on test Aggregate Abrasion Valuesof 13 percent and Polished Stone Values of 64 and 63.

The test methods employed for determining the Po]- ished Stone Valuesand Aggregate Abrasion Values were those specified in B.S.S. (BritishStandard Specification) 812, 1967.

What I claim is:

1. A method of producing a synethic aggregate useful as a road stonehaving a high-skid resistant surface, comprising preparing a finelydivided raw material having the oxides SiO -35 percent, A1 0 15-70percent, FeO 0-65 percent, MgO 0-50 percent, alkalis 5 percent, and CaO5 percent, the combined content of FeO and MgO being sufficient toprovide at least 15 percent of spinels in the finished aggregate,agglomerating the raw material, and performing a single firing of theagglomerated material at a temperature and for a time sufficient tocause recrystallization of the raw materials until substantialelimination of any uncombined magnesium oxide is achieved and a materialis produced comprising spinel crystals imbedded in a matrix of a memberof the group consisting of silicates, and mixtures thereof, and having aporosity low enough to provide an aggregate abrasion value less than 20percent.

2. A method according to claim 1 wherein the A1 0 is in the range 15-40percent, the FeO content in the range 5-40 percent, the MgO content inthe range 5-40 percent, and the combined content of FeO and MgO issufficient to produce at least 25 percent of spinels in the aggregateproduced.

3. A method according to claim 2, in which at least a proportion of theiron content of the raw material is in ferric form, and the firing stepis carried out under reducing conditions.

4. A method according to claim 2, wherein the raw material is a highiron clay.

5. The method of claim 2 wherein the single firing is performed at afirst and second temperature.

6. An improved road stone synthetic aggregate having a highskidresistant surface while being economical to produce having a content asfollows: Si0 5-35 percent, A1 0 15-70 percent, FeO 0-65 percent, MgO0-50 percent, alkalis 5 percent, CaO 5 percent; at least 15 percent ofsaid content being in spinel crystal form, said spinel being in a matrixof silicates and said aggregate having an aggregate abrasion value ofless than 20 percent, and a polished stone value of at least 60.

7. The road stone synthetic aggregate of claim 6 wherein at least 25percent of said content is in spinel form.

2. A method according to claim 1 wherein the Al2O3 is in the range 15-40percent, the FeO content in the range 5-40 percent, the MgO content inthe range 5-40 percent, and the combined content of FeO and MgO issufficient to produce at least 25 percent of spinels in the aggregateproduced.
 3. A method according to claim 2, in which at least aproportion of the iron content of the raw material is in ferric form,and the firing step is carried out under reducing conditions.
 4. Amethod according to claim 2, wherein the raw material is a high ironclay.
 5. The method of claim 2 wherein the single firing is performed ata first and second temperature.
 6. An Improved road stone syntheticaggregate having a highskid resistant surface while being economical toproduce having a content as follows: SiO2 - 5-35 percent, Al2O3 - 15-70percent, FeO - 0-65 percent, MgO - 0-50 percent, alkalis < 5 percent,CaO < 5 percent; at least 15 percent of said content being in spinelcrystal form, said spinel being in a matrix of silicates and saidaggregate having an aggregate abrasion value of less than 20 percent,and a polished stone value of at least
 60. 7. The road stone syntheticaggregate of claim 6 wherein at least 25 percent of said content is inspinel form.